Use of focused accelerated corrosion for metal decommissioning

ABSTRACT

The present invention is directed to a method of using accelerated corrosion to remove metallic assets.

FIELD OF THE INVENTION

This invention relates to a method of accelerated controlled uniformcorrosion of a focused area of a metallic asset.

BACKGROUND OF THE INVENTION

Decommissioning metallic assets that have been impacted with naturallyoccurring hazardous materials which may include mercury, arsenic, leadand other contaminants have been traditionally costly to clean. Thesematerials are often found embedded within the surface scale or corrosionlayer. Standard industry practice to remove these materials can beachieved physically or chemically. The physical approach uses highpressure water jets to detach the surface scale or corrosion from theasset. This process typically generates a significant amount ofwastewater and makes slow progress. The chemical approach commonly usesacidic chemistries that dissolved the inorganic scale or corrosionmaterial. As a result, this process generates a large volume of acidicwaste. U.S. Ser. No. 10/181,706 describes a method to remove hazardousmaterials by accelerating the corrosion process on a focused area of themetallic asset into the surrounding conducting fluid.

Decommissioning of Subsea Pipelines can be prohibitively expensive ifpipeline removal is required. Leaving pipelines in place is often theleast risky alternative from an environmental perspective. StandardIndustry practice during operation is to employ corrosion protection topreserve pipelines and prevent corrosion which may include cathodicprotection-sacrificial anodes and/or impressed current. If removal ofthese types of pipelines and facilities is necessary, then the presenceof impregnated metals and other impurities can add significant costs tothe removal and disposal. Subsea pipelines can be flushed and pigged toremove internal hydrocarbons; however, this process will not removecontaminants impregnated into the metal.

SUMMARY OF THE INVENTION

An embodiment of the invention is controlled uniform corrosion on afocused area of a metallic asset to remove unwanted materials from theasset's surface. The metallic asset is used as a sacrificial anode andcoupled with a cathode assembly of lower redox potential than the asset.A non-conductive permeable spacer, and an electrolytic conducting fluid(e.g., seawater, water containing a corrosion enhancing agent, etc.) areused to complete the circuit between the anode and cathode. A preferredembodiment is an accelerated corrosion on the metal area of up to 0.1 mmthickness in one minute or less.

Another embodiment is a method of accelerating the corrosion of thefocused area by increasing the conducting fluid circulation rate and/ortemperature through the cathode assembly, adjusting the power, morespecifically current, and/or adjusting the spacing between the cathodeand the anode, and/or changing the type of non-conductive permeablespacer. The power source may be DC or AC. When the power supply is a DCpower the amount of power per square meter will be higher than in animpressed system and the power delivered will depend on the number ofbrushes/brush heads employed. A preferred embodiment is 70 amps per 6 cmsquared; 10 up to 50 Watts per cm squared. Another preferred embodimentis to adjust the power in bursts to corrode the metal section inintervals with subsequent scrubbing of the corroded area.

A further embodiment is the use of one or more magnetized moveable brushheads that enables repositioning of the anode for focused corrosion of avariety of areas across the surface of the metallic asset.

A preferred embodiment increases the temperature of the conducting fluiddelivered to the corrosion area up to about 60° C. or alters theelectrolytic chemistry of the solution to increase the reaction rate ofthe corrosion process. The temperature increase or solution chemistrycan be combined with the increase in water circulation to furtheraccelerate the focused corrosion.

A preferred embodiment is to fill the asset with the conducting fluidand submerge the cathode or multiple cathodes a given distance from theasset's surface. In this embodiment 0.1-1 amp/cm2 may be applied overthe course of multiple hours. The asset is then flushed of theconducting fluid and the corrosion material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of corrosion rates increase with increasing currentdensity.

FIG. 2 is a plot of the corrosion rate as a function of treatment time.

FIG. 3 is a plot of steel equivalent corrosion rate v current densityvariable spacer type.

FIG. 4 is a plot of steel equivalent corrosion rate v current densityvariable cathode sizes.

FIG. 5 is a plot of steel equivalent corrosion rate v current densitydegree of corrosion.

FIG. 6 is a plot of power density v time with different spacers.

FIG. 7 is a drawing of an embodiment of the brush pad or brush head fordelivery of conducting fluid.

DETAILED DESCRIPTION OF THE INVENTION

Herein is described a method to remove a contaminant from facilities byfocused uniform accelerated corrosion on an area of the metallic asset.The process as described herein would remove a key waste treatment stepin the state of the art and improve the economy in the treatment ofsurface-contaminated metallic assets, including treatment & handling ofprocess water and could be conducted offshore eliminating the need forcostly on-shore treatment. “Contaminant” as used herein includes anunwanted area of the metallic asset that is targeted for removal.

Unless otherwise indicated, all numbers expressing quantities ofingredients, properties such as molecular weight, reaction conditions,and so forth as used in the specification and claims are to beunderstood as being modified in all instances by the term “about.”Accordingly, unless otherwise indicated, the numerical properties setforth in the following specification and claims are approximations thatmay vary depending on the desired properties sought to be obtained inembodiments of the present invention. Notwithstanding that the numericalranges and parameters setting forth the broad scope of the invention areapproximations, the numerical values set forth in the specific examplesare reported as precisely as possible. Any numerical values, however,inherently contain certain errors necessarily resulting from error foundin their respective measurement.

“Metallic asset” as herein defined comprises an interior section ofpiping, pipeline, tank, separation or pressure vessel, process equipmentand other equipment containing sufficient metal to corrode by theprocess employed herein.

“Conducting fluid” as used herein comprises water containing a corrosionor conductivity enhancing agent.

“Accelerated corrosion” as used herein consists of a corrosion time ofthe focused area of the metallic asset that is equal to or less thanabout one minute.

An embodiment of the invention is controlled uniform corrosion of afocused area of a metallic asset to remove unwanted materials from theasset's surface. The metallic asset is employed as a sacrificial anodeand coupled with a cathode assembly of lower redox potential than theasset comprising a non-conductive permeable spacer the cathode and theanode and an electricity conducting fluid including but not limited toseawater, water containing a corrosion enhancing agent and other fluidswhich are capable of conducting electricity known to one of skill in theart that is used to complete the circuit between the anode and cathode.A preferred embodiment is accelerated corrosion where the metal area ofup to 0.1 mm thickness corrodes in one minute or less.

Another embodiment is a method of accelerating the corrosion of thefocused area by increasing the conducting fluid circulation rate and/ortemperature through the cathode assembly, adjusting the power, morespecifically current, and/or adjusting the spacing between the cathodeand the anode, and/or changing the type of non-conductive permeablespacer. The power source is a DC power supply wherein the amount ofpower per square meter will be higher than in an impressed system andthe power delivered will depend on the number of brushes/brush headsemployed. A preferred embodiment is 70 amps per 6 cm squared; 10 up to50 Watts per cm squared. Another preferred embodiment is to adjust thepower in bursts to corrode the metal section in intervals withsubsequent scrubbing of the corroded area.

A further embodiment is the use of one or more magnetized moveable brushhead or brush pad that enables repositioning of the anode for focusedcorrosion of a variety of areas across the surface of the metallicasset. The brush head or pad is a permeable nonconductive materialaffixed to the bottom of a four cornered plate wherein an irrigationhose is connected into the top of the plate for the flow of a conductingfluid through the brush head or pad. A further embodiment usesnon-conductive rollers as spacers to allow the cathode to freely moveacross the corroding area.

A preferred embodiment is the use of increasing the temperature of theconducting fluid delivered to the corrosion area up to about 60° C. orthe chemical make-up of the solution to increase the reaction rate ofthe corrosion process. The temperature increase or solution chemistrycan be combined with the increase in water circulation to furtheraccelerate the focused corrosion.

A preferred embodiment is to fill the asset with the conducting fluidand submerge the cathode or multiple cathodes a given distance from theasset's surface. In this embodiment 0.1-1 amp/cm2 may be applied overthe course of multiple hours. The asset is then flushed of theconducting fluid and the corrosion material.

Example 1—Electrical Resistance is Reduced as Dry Steel Becomes MoreSaturated During Treatment

4 areas treated sequentially, 2 minutes each area. Treatment began ondry steel surface at Area 1, water flooding of all areas commenced atthe beginning of treatment. Current density can be increased for thesame power usage by pre-wetting the steel.

Pre-Treatment Water Flood Time Current Density Power Density Area (mins)(Amps/cm2) Watts/cm2 1 0 0.63 23.1 2 2 0.62 23.3 3 4 0.73 23.4 4 6 1.0523.9

What is claimed is:
 1. A method for removal of unwanted materials fromthe surface of a metallic asset by controlled uniform corrosion of afocused area of the metallic asset.
 2. The method of claim 1 wherein themetallic asset is employed as a sacrificial anode and coupled with acathode assembly of lower redox potential than the asset comprising anon-conductive permeable spacer the cathode and the anode and anelectricity conducting fluid that is used to complete the circuitbetween the anode and cathode.
 3. The method of claim 2 wherein thefocused area of corrosion is accelerated to affect 0.1 mm thicknesscorrosion in one minute or less.
 4. The method of claim 3 wherein theaccelerating corrosion comprises a) increasing the conducting fluidcirculation rate and/or temperature through the cathode assembly, b)adjusting the power and/or adjusting the spacing between the cathode andthe anode, and/or c) changing the type of non-conductive permeablespacer.
 5. The method of claim 4 wherein the power is sourced by a DCpower supply wherein the amount of power per square meter will be higherthan in an impressed system and the power delivered will be dependentupon a number of brush heads employed to the focused area.
 6. The methodof claim 5 wherein the brush heads are magnetized and moveable.
 7. Themethod of claim 6 wherein the power supply consists of 70 amps per 6 cmsquared, 10 up to 50 Watts per cm squared.
 8. The method of claim 7wherein the power is adjusted in bursts to corrode the metal section inintervals with subsequent scrubbing of the corroded area.
 9. The methodof claim 4 wherein the temperature of the conducting fluid delivered isincreased up to about 60° C.
 10. The method of claim 4 wherein the 0.1-1amp/cm2 is applied is applied through the cathode at a given distancefrom the asset's surface for a period of time sufficient to achievecorrosion.